Electrodes applied to the skin of a patient are commonly employed to non-invasively obtain biopotential signals useful in determining a physiological condition or functioning of a patient. As the anatomy of mammalian skin presents a high electrical impedance and decreases the magnitude and the signal to noise ratio of the biopotential signal obtained by the electrode, it is common to abrade the skin and/or apply electrolytic gel to the skin before using the electrode to improve the electrical characteristics of the signal. See, for example, U.S. Pat. Nos. 5,305,746 and 6,032,064. These steps are time consuming, particularly when a number of electrodes are being applied to the patient. The '746 patent shows a disposable pre-gelled self-prepping electrode which is said to provide good electrical contact while reducing the work of removing the outer layer of skin and wetting the skin with electrolytic gel. The '064 patent shows a disposable pre-gelled, three electrode array. Both the electrode shown in the '746 patent and that shown in the '064 patent will have a finite shelf life as the electrolytic gel will dry out with time.
Another approach to improved electrode performance is to form needles on the electrode that penetrate the outer layer of the skin, thereby to reduce the electrode-skin impedance. See, for example, published PCT patent applications WO 01/52730 and WO 01/52731 and the articles Micromachined Electrodes for Biopotential Measurements, Griss et al., Journal of Microelectromechanical Systems, Vol. 10, No. 1, March 2001 and Characterization of Micromachined Spiked Biopotential Electrodes, Griss et al. in IEEE Transactions on Biomedical Engineering, Vol. 49, No. 6, June 2002. These references describe miniature needles micromachined on the surface of a silicon substrate and covered with a silver/silver chloride layer. The length of the spikes is typically 150 micrometers (μm) and the thickness is approximately 30 μm. See also U.S. Pat. No. 6,334,856 showing a device of similar configuration for use in drug delivery.
A particular, recent application for biopotential signal electrodes is to obtain electroencephalographic (EEG), biopotential signal data for objective quantification of the brain activity of a patient to determine the depth of anesthesia or level of consciousness of the patient. The need for such a determination has become increasingly desirable as the administration of anesthetic agents has become more sophisticated and from the efforts to administer anesthesia that is neither too deep nor too light. Anesthesia that is too light might result in the patient waking up during a surgical or other medical procedure while anesthesia that is too deep wastes expensive drugs and prolongs the recovery period for the patient.
Traditionally, electroencephalographic data has have been obtained by placing a plurality of electrodes on the scalp of the patient at locations specified in a recognized protocol. Applying the electrodes takes time and skill, may require skin preparation, and is especially difficult in areas of the scalp covered with thick hair.
However, in connection with EEG based depth of anesthesia measurements, it has recently been found that for many routine circumstances, it is sufficient to obtain electroencephalographic biopotential signals from only the forehead of the patient. The forehead is hairless and easy to access. This, to some extent, simplifies the application of the electrodes to the patient. However, to minimize the effects of external electrical interference, the electroencephalographic biopotential signals obtained from the patient's forehead or scalp are amplified using differential amplifiers. Such amplifiers have three inputs, a positive signal input, a negative signal input, and a ground input. Hence, the simplest one channel EEG signal measurement requires three electrodes for connection to the differential amplifier. Since it is usually desirable to obtain more than one channel of EEG signal data, numerous electrodes must still be applied to the patient to obtain the desired data so that the application time is not greatly shortened.